Optical concentrator with maximum illumination

ABSTRACT

The radiation concentrator disclosed comprises a frustoconical mirror, or equivalent such as a bundle of tapered optical fibers, which concentrates flux from a frontal concentrator system onto a sensor. The dimensions of the mirror or equivalent are calculated by formulae which maximize the illumination of the sensor while accepting a preselected flux loss.

United States I inventor Pierre Malilaud 95 boulevard Jourdan, Paris,14c, France Appl. No. 832,666

Filed June 12, 1969 Patented Oct. 12, 1971 Priority June 12, 1968 France154,663

OPTICAL CONCENTRATOR WITH MAXIMUM ILLUMINATION [56] References CitedUNITED STATES PATENTS 3,187.627 6/1965 Kapany 350/96 B X 3,413,468 11/1968 Astheimer 350/96 UX 3,467,840 9/ 1969 Weiner 350/96 UX FOREIGNPATENTS 1,1 10,821 4/1968 Great Britain 350/96 Primary Examiner-John K.Corbin Attorney-Schellin & Hoffmann ABSTRACT: The radiation concentratordisclosed comprises a frustoconical mirror, or equivalent such as abundle of tapered optical fibers, which concentrates flux from a frontalconcentrator system onto a sensor. The dimensions of the mirror orequivalent are calculated by formulae which maximize the illumination ofthe sensor while accepting a preselected flux loss.

OPTICAL CONCENTRA'IOR WITH MAXIMUM ILLUMINATION BACKGROUND OF THEINVENTION The present invention relates to radiation concentrator forconcentrating radiation on a sensitive element comprising a firstconcentrator system of relative aperture UN for providing afirst'concentration of the incident flux by forming an image at thesmallest section of a beam whose rays subtend a half angle at the axisof the system of 0, (sin 0, ll2N), and at least one substantiallyfrustoconical element for providing a second concentration of the fluxby internal reflection from its substantially frustoconical surfacewherein the entry section of said substantially frustoconical elementwill be coincident with the image produced by said first concentrationoptical system, and wherein the exit section of said substantiallyfrustoconical element of smaller diameter, will be associated with saidsensitive element.

In French Pat. No. l,$43,l65, of May 6, 1964, a concentrator of theabove mentioned kind is described, which enables maximum possibleillumination power to be achieved whilst respecting the condition thatthe whole of the incident flux is exploited.

SUMMARY OF THE INVENTION The present invention is a development of theinvention the subject of that patent, and concerns the problem ofachieving the maximum illuminating power for the smallest pouible fluxloss and particularly for a flux loss of a predetermined minimumproportion.

The present invention provides a radiation concentrator forconcentrating electromagnetic radiation on a sensitive element,comprising a first concentration system of relative aperture UN forproviding a first concentration of the incident flux by forming an imageat the smallest section of a beam whose rays subtend a half angle 0,, atthe axis of the system (where sin 0, is substantially equal to l/2N),and at least one substantially frustoconical element of apex half angle7 for providing a second concentration of the flux by internalreflection in a medium of refractive index n from its substantiallyfrustoconical surface, wherein the entry section of said substantiallyfrustoconical element, of diameter d,, will be coincident with the imageproduced by said first concentration system, wherein the exit section ofsaid substantially frustoconical element, of smaller diameter d,, willbe associated with said sensitive element through a medium of refractiveindex n,, and wherein:

al sin (It-Y) z in (Bo-v) 7. l-tan 'Y a 1 =1 Mi- 1 tan] I, 1 Q

sin BFSIII (Jr/m; Bo being the half angle of the rays entering saidsubstantially frustoconical mirror after refraction at the entrysection, sin A nJrr, A being the total reflection angle from theenvironment n, to the environment 11,. 1; being a parameter equal to theratio between the flux at the exit section and the flux incident at thethe entry section of said substantially frustoconical element.

From equations (1)and (2) above, for a radiation source of a givenapparent diameter, and for a first concentrator system of given diameterand relative aperture, and selecting a value for the parameter d it ispossible to determine unambiguously those dimensions of thesubstantially frustoconical element which will enable the maximumilluminating power to be achieved whilst losing only the predeterminedproportion of flux.

Other features and advantages of the present invention will be stillbetter understood from a consideration of the following description ofseveral embodiments thereof, given by way ofhexgmple in conjunction withthe accompanying drawings, in w to FIG. I is an optical diagram of aconcentrator in accordance with the invention; and

FIG. 2 is a graph showing the variations in the illumination and theflux at the various sections of a frustoconical mirror.

FIG. I, an optical concentrator device can be seen which combines, onthe one hand, a frontal convergent optical system 1 of relative aperture1 IN, receiving a flux emanating from a distant radiation source whichis not shown and is assumed to be located in a medium of refractiveindex equal to unity, said system producing a real image of the sourcein a plane 2 (focal plane) by means of a convergent beam whose rays 3have an aperture half angle 0,, and, on the other hand, a frustoconicalmirror or equivalent element 4 the entry section of which, of diameterd,, is located in the image plane 2 in coincidence with theaforementioned image, said frustoconi cal mirror having an apex halfangle 7, the refractive index of its internal medium being n, (inrelation to the refractive index of the medium surrounding the radiationsource), and having an exit section of calculated diameter d,, whichexit section is intimately associated with a sensing element 5 of areceiver device, the sensing element 5 being immersed in a medium ofrefractive index n,

In the same FIG. I, the aperture halfangle 0, of the beam of radiationincident on the frustoconical mirror 4 has been illustrated by the lineof a marginal meridian ray 6, and the aperture half angle B, of the ray6' refracted at the entry section :1, of said frustoconical mirror 4.

Choosing a ratio 1; (ratio between the flux picked up at the terminalsection d, and the flux incident at the entry section 11,), andfurthermore, given the characteristics of the frontal convergent opticalsystem I, then it is possible to detennine the dimensions which willunambiguously characterize the frustoconical mirror 4 or equivalentelement by means of the following formulas:

The following numerical example shows how the frustoconical mirror 4 isdesigned in a given instance:

Assume a source, located at a distance, of apparent diameter 5, and afirst convergent optical system constituted by for example a lensobjective of relative aperture 1/].414. This objective has a diameter of55 mm. and a focal distance of 77.8 mm. It produces an image of diameteri from the radiation source, which diameter i is given by: i=77.8- tan5%.8 mm.

The illumination of this image is in a ratio to the theoretical maximumillumination, which,.as those skilled in the art will appreciate, isdefined by I/4 N, in other words H8 in this case.

Assume moreover that it is desired to achieve the maximum illuminationwith direct immersion of the sensing terminal element in a flint glassof index n =n,=l .73, the frustoconical mirror 4 itself being made ofthis same flint glass. This immersion in flint glass is to permit of anextra concentration factor of n,, that is to say three times. In otherwords, it is desired to obtain a (limiting) illumination 24 times morepowerful than with the frontal optical system of relative aperture Ill.4; and at the same time this is to be accompanied by a loss of no morethan say 25 percent of the flux incident at the entry section d,, inother words: 'q'=0.75.

and:

1-4.8 tan 7=Jfi tan y=0.028

we get:

from which;

and

the apex half angle -y=l/36 radians l36') Substituting this value of yin the formula (1), with sin B,= sin /n,=0.354/l .73, in other wordsB,=l 1 48', we ultimately obtain:

6.8/d =0.9996/0.l77l from which: d =l .2 mm. I

The length L of the frustoconical minor is given by (6.8-1.22 tan 7,viz:

lFl00.8 mm.

FIG. 2 illustrates a graph summarizing the results obtained by theconcentrator, in the case of the numerical example hereinbefore quoted.0n the abscissae, the length L of the frustoconical mirror between theentry section d, and an arbitrary section 11,, has been plotted, themirror having an apex half angle of -y=l/36 radians. Calling k the ratiobetween the diameter d, and the diameter d, of an arbitrary section, we

have:

1 1 l L 2 tria I?) E The ordinates are chosen, on the one hand so thatthe curve 7 represents the variations in the ratio between theillumination at an arbitrary section and the illumination at the entrysection (1,. On the other hand the curve 8 represents the variation inthe ratio between the flux reaching an arbitrary section d, and the flux1 incident at the entry section 11,.

From a consideration of these curves, it will be clearly seen how, inrespect of a frustoconical mirror, the system can be optimized in twoways. The first of these ways, at a section d makes it possible toachieve the strongest illumination (here 70 percent of the maximumillumination) without loss of flux. This optimization is obtained bydimensioning a frustoconical mirror in accordance with the disclosure ofFrench Pat. No. l,543,l65 of May 6, 1964, and does not form part of thepresent invention.

The second way in which the system can be optimized, is offered by thepresent invention and is embodied bythe section d defined in accordancewith the present invention, and makes it possible to achieve the maximumlimiting illumination for the least possible flux loss (here 25percent). This optimization is achieved through the dimensioning whichforms the object of the present patent application.

It should be pointed out at this juncture that the formulas l and 2hereinbefore listed are general formulas and several special cases willbe examined hereinafter.

In addition, it has been found advantageous, where n, differs from unityand where n, differs from both n, and unity, to

adjust the parameters 7 and B, in order to satisfy the relationship:

ZK y

K being a whole number.

bine to produce the maximum limiting illumination.

V I In special cases, the general formulas l and 2, may be simp fies lnthe case wifie n n, and are different from unity (this being the casewhere the sensing element is immersed directly in a refractivefrustoconical mirror), and taking account of the fact that in this case:

sin A=n,/n =l;

)t=rr/2 and sin (1r/2 -7) =cos -y the formulae become:

(1 cos 7 z Sin (Bo-v) 1 I si a. (5)

Finally, in the case of a hollow frustoconical mirror, where both n andn, are equal to unity and taking into account the fact that:

sin [3,, sin0 ln sin 9, and therefore fi 0, and n /n,= l

It is possible to associate with a common frontal concentrator systemseveral elementary frustoconical mirrors assembled in a bundle andoperating in parallel, the bunch being made up for example offrustoconical optical fibers each having dimensions calculated inaccordance with equations (l) and (2) above. Several advantages ariseout of this kind of system. For example, it is possible in this way toreproduce upon the sensing element a mosaic image of the radiationsource. The length of the bunch of elementary frustoconical mirrors thusproduced is shorter in the proportion of the square root of the numberof elementary mirrors, considered in a section passing through the axis,than the length of the equivalent single frustoconical mirror.

The present invention has applications in various fields, for/ examplethe detection of radiation of a wide range of wavelengths, in particularin the infrared spectrum; surveillance; optical telecommunications;monitoring of hot environments, and flame monitoring; photocontrol andthennocontrol; light condensers; photographic concentrators; solarenergy concentrators; laser supplies.

lclaim:

1. A radiation concentrator for concentrating electromagnetic radiationon a sensitive element, comprising a first concentration system ofrelative aperture UN for providing s first concentration of the incidentflux by forming an image at the smallest section of a beam whose rayssubtend a half angle 0,, at the axis of the system (where sin 1 0, issubstantially equal to H2 N), and at least one substantiallyfrustoconical element of apex half angle 7 for providing a secondconcentration of the flux by internal reflection in a medium ofrefractive index n from its substantially frustoconical surface, whereinthe entry section of said substantially frustoconical element, ofdiameter d,, will be coincident with the image produced by said firstconcentration system, wherein the exit section of said substantiallyfrustoconical element, of smaller diameter :1, will be associated withsaid sensitive element through a medium of refractive index n,, andwherein:

flux incident at the entry section of the said substantiallyfrustoconical element 2. A concentrator as claimed in claim 1. wherein:A-B, 2 2K 7 ..(3) 5 K being a whole number.

1. A radiation concentrator for concentrating electromagnetic radiationon a sensitive element, comprising a first concentration system ofrelative aperture 1/N for providing s first concentration of theincident flux by forming an image at the smallest section of a beamwhose rays subtend a half angle theta 1, at the axis of the system(where sin 1 theta 1 is substantially equal to 1/2 N), and at least onesubstantially frustoconical element of apex half angle gamma forproviding a second concentration of the flux by internal reflection in amedium of refractive index n1 from its substantially frustoconicalsurface, wherein the entry section of said substantially frustoconicalelement, of diameter d1, will be coincident with the image produced bysaid first concentration system, wherein the exit section of saidsubstantially frustoconical element, of smaller diameter d3, will beassociated with said sensitive element through a medium of refractiveindex n2, and wherein:
 2. A concentrator as claimed in claim 1, wherein:lambda - Beta o 2K gamma .......(3) K being a whole number.